- Email: [email protected]
CAM crowns
journal of dentistry 38 (2010) 995–1000
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/jden
Two imaging techniques for 3D quantification of pre-cementation space for CAD/CAM crowns Patchanee Rungruanganunt a,*, J. Robert Kelly b, Douglas J. Adams c a
Department of Reconstructive Sciences, University of Connecticut Health Center, 263 Farmington Avenue, Farmington, CT 06030-1615, USA Department of Reconstructive Sciences, Center for Biomaterials, University of Connecticut Health Center, Farmington, CT, USA c Department of Orthopaedic Surgery, New England Musculoskeletal Institute, University of Connecticut Health Center, Farmington, CT, USA b
article info
abstract
Article history:
Objectives: Internal three-dimensional (3D) ‘‘fit’’ of prostheses to prepared teeth is likely
Received 29 April 2010Received in
more important clinically than ‘‘fit’’ judged only at the level of the margin (i.e. marginal
revised form
‘‘opening’’). This work evaluates two techniques for quantitatively defining 3D ‘‘fit’’, both
18 August 2010Accepted 23 August
using pre-cementation space impressions: X-ray microcomputed tomography (micro-CT)
2010
and quantitative optical analysis. Both techniques are of interest for comparison of CAD/ CAM system capabilities and for documenting ‘‘fit’’ as part of clinical studies. Methods: Pre-cementation space impressions were taken of a single zirconia coping on its
Keywords:
die using a low viscosity poly(vinyl siloxane) impression material. Calibration specimens of
Crown
this material were fabricated between the measuring platens of a micrometre. Both
CAD/CAM
calibration curves and pre-cementation space impression data sets were obtained by
Cement space
examination using micro-CT and quantitative optical analysis. Regression analysis was
Pre-cementation space
used to compare calibration curves with calibration sets.
Cement thickness
Results: Micro-CT calibration data showed tighter 95% confidence intervals and was able to
Fit
measure over a wider thickness range than for the optical technique. Regions of interest
Micro-CT
(e.g., lingual, cervical) were more easily analysed with optical image analysis and this
Internal adaptation
technique was more suitable for extremely thin impression walls (<10–15 mm). Specimen preparation is easier for micro-CT and segmentation parameters appeared to capture dimensions accurately. Conclusions: Both micro-CT and the optical method can be used to quantify the thickness of pre-cementation space impressions. Each has advantages and limitations but either technique has the potential for use as part of clinical studies or CAD/CAM protocol optimization. # 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
Many automated systems are emerging for the fabrication of dental prostheses. As yet there are no standards for the fit of these prostheses or protocols for measuring fit. There are also no thoughtful or evidence-based criteria for defining ‘‘wellfitting.’’ For example, the complete three-dimensional (3D) description of the relationship between prostheses and
prepared teeth likely provides more clinically meaningful information than commonly accepted measurements taken at the level of the margin (i.e. marginal ‘‘opening’’). Based on published literature it is far easier to make the case that marginal openings do not create serious problems than it is to argue that larger openings equate with a poor clinical outcome.1–3 Marginal opening of fixed prostheses has only been associated with marginal gingivitis (gingival index and
* Corresponding author. Tel.: +1 860 679 4175; fax: +1 860 679 1370. E-mail address: [email protected] (P. Rungruanganunt). 0300-5712/$ – see front matter # 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2010.08.015
996
journal of dentistry 38 (2010) 995–1000
crevicular fluid volume, but not pocket depth)1 and do not appear to be predictive of clinical longevity. Similarly, gap widths for restorations have no direct relationship with secondary caries.2,3 Occlusal misfit, however, and the resultant increase in cement thickness may have a significant influence on structural durability of all-ceramic crowns (unpublished finite element and in vitro research demonstrates 2 decrease in load-bearing of molar crowns at 500 mm versus 50 mm with bonded cement, University of Connecticut, November, 2009). Additionally, axial wall cement space can influence the retention of fixed prostheses, with too thin of cementation space reported to reduce retention.4,5 Three-dimensional relationships can be approximately described by data taken from serial cross-sections of cemented prostheses.6–8 Although inherently destructive and timeconsuming, this technique is adequately so long as cement space dimensions do not change rapidly over the slice-to-slice distances used and slices randomly select any systematic anomalies. However, serial sectioning of cemented prostheses remains an in vitro protocol not amenable for use as part of a clinical trial (e.g., in documenting ‘‘fit’’ versus longevity). Prosthesis fit can be recorded in three dimensions by making an impression of the ‘‘pre-cementation space’’ using low viscosity impression materials. This technique is now commonly used by clinicians (without removal of the impression) as a guide to where material should be removed to improve seating of fixed restorations.9 The impression replica technique has been modified for use in research studies to measure pre-cementation space.10,11 It was also used for 3D mapping in earlier work by a novel photometric technique.12 Since the chromophore (light absorbing dye) in impression materials is very uniformly distributed (and of constant concentration with careful mixing), light absorption can be directly related to material thickness.12 The amount of light transmitted through the impression material is proportional to the known thickness, via a power law relationship, known as the Beer–Lambert Law.13 The advent of high resolution X-ray computed tomography (micro-CT) presents a potential alternative technique for three-dimensional evaluation of pre-cementation space due to its ability to acquire 3D relationships between features having sufficiently different contrast with a resolution of a few micrometres. This study examined the two aforementioned imaging techniques. Both utilize imaging of poly(vinyl siloxane) impressions to compare their potential for mapping the precementation space. (1) Direct 3D data collection using microCT imaging and (2) optical density image analysis.
2.
Materials and methods
2.1.
Micro-CT imaging and analysis
A master die and zirconia coping of a maxillary central incisor were provided by the manufacturer (Lava, 3M ESPE, Seefeld, Germany). The Lava1 coping was filled with ultra-light body poly(vinyl siloxane) impression material (Express, 3M ESPE, St. Paul, MN, USA) inserted on the master die with firm finger pressure (approximately 20 N), and then immediately placed under a 22.24 N (5 lb) static load for 10 min.14 The Lava1
coping, with the silicone film attached, was removed from the master die. Soft red boxing wax was gently placed into the die cavity to function as a radiolucent support, replacing the die (being too radiopaque). The Lava1 coping was then carefully removed, leaving the silicone film on the soft red boxing wax. Standardized thickness specimens were fabricated from the same impression material (10 mm, 20 mm, 25 mm, 40 mm, 50 mm, 60 mm, and 100 mm) between the measuring plates of a micrometre (Model No. 734, L.S. Starrett Co., Athol, MA, USA). A thin rim of material was maintained around each specimen to allow for positioning within the micro-CT fixture. The specimen and standards were imaged using microfocus conebeam X-ray computed tomography (mCT, Scanco Medical AG, Bruttisellen, Switzerland) to determine whether it was possible to visualize the silicone film and measure its dimensions in 3D. Measured thicknesses of the standards were compared to known thicknesses using linear regression (TableCurve 2D, v. 5.01, Systat Software, Inc., Richmond, CA, USA). Imaging was performed at 45 kV and 177 mA, collecting 2000 projections per rotation at 300 ms integration time to provide 8 mm resolution (scan time of 13 min per millimetre). Three-dimensional images were reconstructed using standard convolution and back-projection algorithms with Shepp and Logan filtering, and rendered within a 16.4 mm field-of-view at a discrete density of 1,953,125 voxels/mm3 (isometric 8 micrometre voxels). Threshold segmentation of scaffold from background was performed in conjunction with a constrained Gaussian filter to reduce noise.
2.2.
Quantitative optical analysis
The original concept of measuring cement space thickness by light transmission through a coloured impression material (utilizing the Beer–Lambert relationship) was described by Kelly et al.12 This current technique utilized the same concept but included the fabrication of a calibration set of impression materials, simultaneous digital photography of these thickness standards, and the transilluminated pre-cementation space impression. Data from the calibration set was curve-fit within an image analysis program that allowed mapping of the pre-cementation space impression. These were separated into seven thickness groups as a percentage of surface area and visualized as colour-coded thickness maps. Standardized specimen thicknesses (10 mm, 15 mm, 20 mm, 25 mm, 70 mm, and 100 mm) again were formed of the same impression material used for micro-CT with the same micrometre (in this case the peripheral rim being removed). These thickness standards were later used to calibrate the images of the impression specimen obtained for measurement. One precementation space impression (Lava coping on its die) was fabricated as for the micro-CT technique. The pre-cementation space impression was cut at the interproximal and incisal line angles in order to be placed flat on a glass microscope slide (i.e. an impression ‘‘fillet’’), and covered with a second glass slide to stabilize the specimen. Calibration specimens were similarly housed between two glass slides and both placed on a light box containing uniform colour-corrected fluorescent bulbs (Kaiser Prolite 5000, B&H Photo, New York City, NY, USA). The precementation space impression and adjacent calibration specimens were photographed using a digital camera (Nikon
journal of dentistry 38 (2010) 995–1000
[(Fig._1)TD$IG] FinePix S1 Pro, Tokyo, Japan). Although the impression material was coloured, the amount of transmitted light was converted to 8-bit grey levels (values) for optical density analysis (MetaMorph1 Image Analysis Software Version 4.0, Universal Imaging Corp. Downingtown, PA, USA). From these sample greyscale intensities for a given bit-depth and thickness range, a calibration curve was constructed (which was later statistically extrapolated to widen the range of all possible intensity values). The non-linear calibration curve was applied in order to convert the greyscale intensities (pixel values) to micrometres. This grey level calibration curve could be applied to all objects being imaged simultaneously (subsequent images would also then contain the calibration thickness set). For 8-bit images, the scale ranges from the greyscale value 0, which represents black, to value 255, which represents white. Segmented histogram analysis was then carried out by dividing image greyscale values into histogram bins (i.e. thickness groups). For this analysis, a total of seven bins were used, with each bin represented by a different colour (transparent, red, green, blue, cyan, magenta, and black) denoting a thickness range. The highest intensity (value) bin was set to the colour ‘‘transparent’’ which was seen as ‘‘showthrough’’, representing thickness values less than 10 mm. The lowest intensity bin was set to the colour ‘‘black’’ (representing thickness values greater than 100 mm). MetaMorph1 allowed the creation of custom reference regions (i.e. defining the measurement area or ‘‘cell’’); this capability was used to define distinct portions of the tooth as identified in Section 3.15 Image analysis then provided information on the total pixels (number of pixels in the image), cell area (total of all pixels in the custom reference region), bin area (number of pixels in the selected bin), and % cell area (selected bins percentage of the cell area) for each region defined. Measured thicknesses of the standards were compared to known thicknesses using linear regression (TableCurve 2D, v. 5.01, Systat Software, Inc., Richmond, CA, USA).
3.
Results
3.1.
Micro-CT analysis
Micro-CT was found to clearly differentiate the silicone film from its wax support, as seen in the cross-sectional view in Fig. 1. Hence, with the micro-CT technique the cement space can be measured throughout its full three-dimensional volume (Fig. 1). These three-dimensional data sets can be used for both quantitative descriptions of the impression replica as well as for visualization by specifying thickness as a function of colour (Fig. 2). Linear regression of the known calibration thicknesses versus micro-CT output appears in Fig. 3 (TableCurve 2D, v. 5.01, Systat Software, Inc., Richmond, CA, USA). Confidence intervals (95%) are very tight and the linear correlation is strong with a slope of 0.967 (r2 = 0.99). The distribution of thicknesses in the pre-cementation space replica is represented in Fig. 4 as volume percentages in each of the 12 thickness groups.
997
Fig. 1 – 2D slice of pre-cementation space impression material (magnification bar = 1 mm).
3.2.
Quantitative optical analysis
Optical density data (as pixel values) versus calibration set thicknesses appear in Fig. 5 along with the calibration curve-fit by the MetaMorph1 software. As expected, this relationship is a natural logarithmic function of thickness (r2 = 0.95). This data was curve-fit (to the same equation used by MetaMorph1) to generate the 95% confidence intervals in Fig. 5 (TableCurve 2D). Fig. 6 shows the ‘‘raw’’ optical image of the cementation space ‘‘fillet’’ for orientation and the corresponding MetaMorph1 colour thickness colour map. Four different regions of clinical interest were identified (see Fig. 6 caption) that also
[(Fig._2)TD$IG]
Fig. 2 – 3D representation of pre-cementation space impression material.
998
journal of dentistry 38 (2010) 995–1000
[(Fig._3)TD$IG]
[(Fig._5)TD$IG]
Fig. 3 – Linear regression analysis of micro-CT thickness measurements versus known thicknesses from micrometre.
[(Fig._4)TD$IG]
Fig. 4 – Pre-cementation space impression divided into 12 thickness groups, as measured by micro-CT, by the volume percentage for each group.
appeared to have different thicknesses. These four areas were used for statistical and comparative analysis. Fig. 7 (comparable to Fig. 4 for micro-CT) shows the overall distribution of thickness in six groups. The optical analysis with MetaMorph1 easily allowed regions to be defined which provided interesting additional information as examined in Section 4. Cumulative percent area by thickness for these four regions is graphed in Fig. 8.
4.
Discussion
The results of this investigation indicate that both techniques can be used to quantitatively measure impression replicas for pre-cementation space, each having limitations and advantages. An overview comparing the characteristics and capabilities of both techniques is presented in Table 1. In general,
Fig. 5 – Calibration curve of known thickness (mm) versus transmitted light (pixels values). This logarithmic function based on this calibration curve was applied to pixel values measured from the pre-cementation space ‘‘fillet’’ to generate optical thickness data (MetaMorphW).
micro-CT measurements were characterized by much tighter 95% confidence intervals than found for the optical imaging method (Fig. 3 versus Fig. 5) and was capable of collecting data over a larger thickness range. The different results (ranges) in Fig. 4 versus Fig. 7 likely resulted from true thickness differences in the two replicas rather than from technique variations, since both techniques were independently validated with micrometre-fabricated calibration sets. Whilst the optical technique is probably superior below 15 mm, its upper limit was not much greater than 100 mm and the ability to distinguish amongst specimens decreased towards this upper limit (e.g., plateau in Fig. 5 and increasing width of 95% confidence boundaries). Pixel values used over thicknesses between 10 mm and 100+ mm ranged only between 110 and 125 (of the 0–255 full scale). The range of values available for distinguishing thickness may have been somewhat limited since the camera used 8-bit image coding and all pixel data were collapsed into greyscale data. Increasing the range of values coding for thickness might tighten confidence intervals, allow for higher discrimination, and broaden the measurable thickness range. This might be possible using a 16-bit B&W camera instead of the 8-bit colour camera utilized in this work. MetaMorph1 Imaging System software can be used more easily to identify specific areas for thickness, such as those labelled in Fig. 6b. In addition, all regions are automatically orthogonal (i.e. 908 to the slice plane). The colour-coded image from MetaMorph1 provided a valuable qualitative visual sense of thickness variations by area and these areas can be separately used for calculating mean statistics. The technique of ‘‘filleting’’ the specimen so that it was held completely flat between microscope slides for measurement may become problematic when preparing bicuspid and molar specimens. In addition, the calibration set must be imaged alongside for this technique.
journal of dentistry 38 (2010) 995–1000
[(Fig._6)TD$IG]
999
Fig. 6 – (a) Anatomy of a cementation space ‘‘fillet’’ for orientation (cut made on insical edge extended down mesial and distal interproximal surfaces). (b) Regions of interest as defined for measurement: 1 = cervical third of crown; 2 = lingual slope between cervical third and lingual fossa; 3 = incisal 2/3 of labial wall; 4 = lingual fossa.
[(Fig._7)TD$IG]
[(Fig._8)TD$IG]
Fig. 7 – Pre-cementation space impression divided into six thickness groups, as measured by MetaMorphW, by the surface area percentage for each group.
The micro-CT technique provides large volumetric data sets which can be used for research and teaching purposes. Thickness measurements and mean statistics typically are automatically available in accompanying analysis software.
Fig. 8 – Cumulative area by thickness for each of the four pre-cementation space areas defined above in Fig. 6b.
The result is highly precise over a wide thickness range. One thickness validation set per impression material is all that should be required to establish segmentation parameters, and is capable of resolving thickness to approximately 10 mm.
Table 1 – Summary of characteristics and capabilities of the two techniques. Micro-CT Digital 3D data set Only one calibration set needed for validation of thickness data per impression material Mean statistics are automatically available Specimen preparation is similar for any tooth Can measure precisely over wider thickness range (10 mm to >1 mm) Requires instrument availability
Quantitative optical analysis All regions are automatically quantified (due to physical ‘‘filleting’’ of specimens) Calibration set required in field-of-view for each specimen Mean statistics need to be calculated separately from pixels-per-thickness data for each region of interest Specimen preparation is more extensive for posterior teeth Can measure thicknesses less than 10 mm but upper limit of approximately 100 mm (depending on optical density of impression material, intensity of light source and digital camera characteristics) Requires imaging software availability
1000
journal of dentistry 38 (2010) 995–1000
Micro-CT data provides opportunities for dramatic 3D visualization. Micro-CT can be expensive and time-consuming regarding both scanning time for high spatial resolution needed as well as for volumetric analysis. Both techniques require either technician assistance or a learning curve for optimum software utilization.
4.
5.
6.
5.
Conclusions
Both micro-CT and an optical imaging method can be used to quantify the thickness of pre-cementation space impressions. Each has advantages and limitations but either technique has the potential for use as part of clinical studies or research into CAD/CAM protocol optimization.
Acknowledgement Authors gratefully acknowledge financial and materials support from 3M ESPE (Seefeld, Germany) that allowed this comparative analysis project to be undertaken.
7.
8.
9.
10.
11.
references 12. 1. Felton DA, Kanoy BE, Bayne SC, Wirthman GP. Effect of in vivo crown margin discrepancies on periodontal health. Journal of Prosthetic Dentistry 1991;65:357–64. 2. Mjo¨r IA, Toffenetti F. Secondary caries: a literature review with case reports. Quintessence International 2000;31:165–79. 3. So¨derholm K-J, Antonson DE, Fischlschweiger W. Correlation between marginal discrepancies at the amalgam/tooth interface and recurrent caries. In: Anusavice
13. 14.
15.
KA, editor. Quality evaluations of dental restorations. Chicago: Quintessence Books; 1989. p. 95–110. Marker VA, Miller AW, Miller BH, Swepston JH. Factors affecting the retention and fit of gold castings. Journal of Prosthetic Dentistry 1987;57:425–30. Passon C, Lambert RH, Lambert RL, Newman S. The efect of multiple layers of die-spacer on crown retention. Operative Dentistry 1992;17:42–9. Brukl CE, Philp GK. The fit of molded all-ceramic, twin foil, and conventional ceramic crowns. Journal of Prosthetic Dentistry 1987;58:408–13. Nakamura T, Nonaka M, Maruyama T. In vitro fitting accuracy of copy-milled alumina cores and all-ceramic crowns. International Journal of Prosthodontics 2000;13: 189–93. Suarez MJ, Villaumbrosia PG, Pradies G, Lozano JFL. Comparison of the marginal fit of Procera AllCeram crowns with two finish lines. International Journal of Prosthodontics 2003;16:229–32. Davis SH, Kelly JR, Campbell SD. Use of an elastomeric material to improve the occlusal seat and marginal seal of cast restorations. Journal of Prosthetic Dentistry 1989;66: 288–329. Coli P, Karlsson S. Fit of a new pressure-sintered zirconium dioxide coping. International Journal of Prosthodontics 2004;17:59–64. Laurent M, Scheer P, Dejou J, Laborde G. Clinical evaluation of the marginal fit of cast crowns validation of the silicone replica method. Journal of Oral Rehabilitation 2008;35:116–22. Kelly JR, Davis SH, Campbell SD. Non-destructive, 3-D internal fit mapping of fixed prostheses. Journal of Prosthetic Dentistry 1989;61:368–73. Levine IN. Physical chemistry. 2nd ed. New York, NY: McGraw-Hill Book Co.; 1983. p. 682. Brukl CE, Philip GK. The fit of molded all-ceramic, twin foil, and conventional ceramic crowns. Journal of Prosthetic Dentistry 1987;58:408–13. MetaMorph1 Imaging System: Task Guide. Version 4.0 for Microsoft Windows 951. Universal Imaging Corporation.
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/jden
Two imaging techniques for 3D quantification of pre-cementation space for CAD/CAM crowns Patchanee Rungruanganunt a,*, J. Robert Kelly b, Douglas J. Adams c a
Department of Reconstructive Sciences, University of Connecticut Health Center, 263 Farmington Avenue, Farmington, CT 06030-1615, USA Department of Reconstructive Sciences, Center for Biomaterials, University of Connecticut Health Center, Farmington, CT, USA c Department of Orthopaedic Surgery, New England Musculoskeletal Institute, University of Connecticut Health Center, Farmington, CT, USA b
article info
abstract
Article history:
Objectives: Internal three-dimensional (3D) ‘‘fit’’ of prostheses to prepared teeth is likely
Received 29 April 2010Received in
more important clinically than ‘‘fit’’ judged only at the level of the margin (i.e. marginal
revised form
‘‘opening’’). This work evaluates two techniques for quantitatively defining 3D ‘‘fit’’, both
18 August 2010Accepted 23 August
using pre-cementation space impressions: X-ray microcomputed tomography (micro-CT)
2010
and quantitative optical analysis. Both techniques are of interest for comparison of CAD/ CAM system capabilities and for documenting ‘‘fit’’ as part of clinical studies. Methods: Pre-cementation space impressions were taken of a single zirconia coping on its
Keywords:
die using a low viscosity poly(vinyl siloxane) impression material. Calibration specimens of
Crown
this material were fabricated between the measuring platens of a micrometre. Both
CAD/CAM
calibration curves and pre-cementation space impression data sets were obtained by
Cement space
examination using micro-CT and quantitative optical analysis. Regression analysis was
Pre-cementation space
used to compare calibration curves with calibration sets.
Cement thickness
Results: Micro-CT calibration data showed tighter 95% confidence intervals and was able to
Fit
measure over a wider thickness range than for the optical technique. Regions of interest
Micro-CT
(e.g., lingual, cervical) were more easily analysed with optical image analysis and this
Internal adaptation
technique was more suitable for extremely thin impression walls (<10–15 mm). Specimen preparation is easier for micro-CT and segmentation parameters appeared to capture dimensions accurately. Conclusions: Both micro-CT and the optical method can be used to quantify the thickness of pre-cementation space impressions. Each has advantages and limitations but either technique has the potential for use as part of clinical studies or CAD/CAM protocol optimization. # 2010 Elsevier Ltd. All rights reserved.
1.
Introduction
Many automated systems are emerging for the fabrication of dental prostheses. As yet there are no standards for the fit of these prostheses or protocols for measuring fit. There are also no thoughtful or evidence-based criteria for defining ‘‘wellfitting.’’ For example, the complete three-dimensional (3D) description of the relationship between prostheses and
prepared teeth likely provides more clinically meaningful information than commonly accepted measurements taken at the level of the margin (i.e. marginal ‘‘opening’’). Based on published literature it is far easier to make the case that marginal openings do not create serious problems than it is to argue that larger openings equate with a poor clinical outcome.1–3 Marginal opening of fixed prostheses has only been associated with marginal gingivitis (gingival index and
* Corresponding author. Tel.: +1 860 679 4175; fax: +1 860 679 1370. E-mail address: [email protected] (P. Rungruanganunt). 0300-5712/$ – see front matter # 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2010.08.015
996
journal of dentistry 38 (2010) 995–1000
crevicular fluid volume, but not pocket depth)1 and do not appear to be predictive of clinical longevity. Similarly, gap widths for restorations have no direct relationship with secondary caries.2,3 Occlusal misfit, however, and the resultant increase in cement thickness may have a significant influence on structural durability of all-ceramic crowns (unpublished finite element and in vitro research demonstrates 2 decrease in load-bearing of molar crowns at 500 mm versus 50 mm with bonded cement, University of Connecticut, November, 2009). Additionally, axial wall cement space can influence the retention of fixed prostheses, with too thin of cementation space reported to reduce retention.4,5 Three-dimensional relationships can be approximately described by data taken from serial cross-sections of cemented prostheses.6–8 Although inherently destructive and timeconsuming, this technique is adequately so long as cement space dimensions do not change rapidly over the slice-to-slice distances used and slices randomly select any systematic anomalies. However, serial sectioning of cemented prostheses remains an in vitro protocol not amenable for use as part of a clinical trial (e.g., in documenting ‘‘fit’’ versus longevity). Prosthesis fit can be recorded in three dimensions by making an impression of the ‘‘pre-cementation space’’ using low viscosity impression materials. This technique is now commonly used by clinicians (without removal of the impression) as a guide to where material should be removed to improve seating of fixed restorations.9 The impression replica technique has been modified for use in research studies to measure pre-cementation space.10,11 It was also used for 3D mapping in earlier work by a novel photometric technique.12 Since the chromophore (light absorbing dye) in impression materials is very uniformly distributed (and of constant concentration with careful mixing), light absorption can be directly related to material thickness.12 The amount of light transmitted through the impression material is proportional to the known thickness, via a power law relationship, known as the Beer–Lambert Law.13 The advent of high resolution X-ray computed tomography (micro-CT) presents a potential alternative technique for three-dimensional evaluation of pre-cementation space due to its ability to acquire 3D relationships between features having sufficiently different contrast with a resolution of a few micrometres. This study examined the two aforementioned imaging techniques. Both utilize imaging of poly(vinyl siloxane) impressions to compare their potential for mapping the precementation space. (1) Direct 3D data collection using microCT imaging and (2) optical density image analysis.
2.
Materials and methods
2.1.
Micro-CT imaging and analysis
A master die and zirconia coping of a maxillary central incisor were provided by the manufacturer (Lava, 3M ESPE, Seefeld, Germany). The Lava1 coping was filled with ultra-light body poly(vinyl siloxane) impression material (Express, 3M ESPE, St. Paul, MN, USA) inserted on the master die with firm finger pressure (approximately 20 N), and then immediately placed under a 22.24 N (5 lb) static load for 10 min.14 The Lava1
coping, with the silicone film attached, was removed from the master die. Soft red boxing wax was gently placed into the die cavity to function as a radiolucent support, replacing the die (being too radiopaque). The Lava1 coping was then carefully removed, leaving the silicone film on the soft red boxing wax. Standardized thickness specimens were fabricated from the same impression material (10 mm, 20 mm, 25 mm, 40 mm, 50 mm, 60 mm, and 100 mm) between the measuring plates of a micrometre (Model No. 734, L.S. Starrett Co., Athol, MA, USA). A thin rim of material was maintained around each specimen to allow for positioning within the micro-CT fixture. The specimen and standards were imaged using microfocus conebeam X-ray computed tomography (mCT, Scanco Medical AG, Bruttisellen, Switzerland) to determine whether it was possible to visualize the silicone film and measure its dimensions in 3D. Measured thicknesses of the standards were compared to known thicknesses using linear regression (TableCurve 2D, v. 5.01, Systat Software, Inc., Richmond, CA, USA). Imaging was performed at 45 kV and 177 mA, collecting 2000 projections per rotation at 300 ms integration time to provide 8 mm resolution (scan time of 13 min per millimetre). Three-dimensional images were reconstructed using standard convolution and back-projection algorithms with Shepp and Logan filtering, and rendered within a 16.4 mm field-of-view at a discrete density of 1,953,125 voxels/mm3 (isometric 8 micrometre voxels). Threshold segmentation of scaffold from background was performed in conjunction with a constrained Gaussian filter to reduce noise.
2.2.
Quantitative optical analysis
The original concept of measuring cement space thickness by light transmission through a coloured impression material (utilizing the Beer–Lambert relationship) was described by Kelly et al.12 This current technique utilized the same concept but included the fabrication of a calibration set of impression materials, simultaneous digital photography of these thickness standards, and the transilluminated pre-cementation space impression. Data from the calibration set was curve-fit within an image analysis program that allowed mapping of the pre-cementation space impression. These were separated into seven thickness groups as a percentage of surface area and visualized as colour-coded thickness maps. Standardized specimen thicknesses (10 mm, 15 mm, 20 mm, 25 mm, 70 mm, and 100 mm) again were formed of the same impression material used for micro-CT with the same micrometre (in this case the peripheral rim being removed). These thickness standards were later used to calibrate the images of the impression specimen obtained for measurement. One precementation space impression (Lava coping on its die) was fabricated as for the micro-CT technique. The pre-cementation space impression was cut at the interproximal and incisal line angles in order to be placed flat on a glass microscope slide (i.e. an impression ‘‘fillet’’), and covered with a second glass slide to stabilize the specimen. Calibration specimens were similarly housed between two glass slides and both placed on a light box containing uniform colour-corrected fluorescent bulbs (Kaiser Prolite 5000, B&H Photo, New York City, NY, USA). The precementation space impression and adjacent calibration specimens were photographed using a digital camera (Nikon
journal of dentistry 38 (2010) 995–1000
[(Fig._1)TD$IG] FinePix S1 Pro, Tokyo, Japan). Although the impression material was coloured, the amount of transmitted light was converted to 8-bit grey levels (values) for optical density analysis (MetaMorph1 Image Analysis Software Version 4.0, Universal Imaging Corp. Downingtown, PA, USA). From these sample greyscale intensities for a given bit-depth and thickness range, a calibration curve was constructed (which was later statistically extrapolated to widen the range of all possible intensity values). The non-linear calibration curve was applied in order to convert the greyscale intensities (pixel values) to micrometres. This grey level calibration curve could be applied to all objects being imaged simultaneously (subsequent images would also then contain the calibration thickness set). For 8-bit images, the scale ranges from the greyscale value 0, which represents black, to value 255, which represents white. Segmented histogram analysis was then carried out by dividing image greyscale values into histogram bins (i.e. thickness groups). For this analysis, a total of seven bins were used, with each bin represented by a different colour (transparent, red, green, blue, cyan, magenta, and black) denoting a thickness range. The highest intensity (value) bin was set to the colour ‘‘transparent’’ which was seen as ‘‘showthrough’’, representing thickness values less than 10 mm. The lowest intensity bin was set to the colour ‘‘black’’ (representing thickness values greater than 100 mm). MetaMorph1 allowed the creation of custom reference regions (i.e. defining the measurement area or ‘‘cell’’); this capability was used to define distinct portions of the tooth as identified in Section 3.15 Image analysis then provided information on the total pixels (number of pixels in the image), cell area (total of all pixels in the custom reference region), bin area (number of pixels in the selected bin), and % cell area (selected bins percentage of the cell area) for each region defined. Measured thicknesses of the standards were compared to known thicknesses using linear regression (TableCurve 2D, v. 5.01, Systat Software, Inc., Richmond, CA, USA).
3.
Results
3.1.
Micro-CT analysis
Micro-CT was found to clearly differentiate the silicone film from its wax support, as seen in the cross-sectional view in Fig. 1. Hence, with the micro-CT technique the cement space can be measured throughout its full three-dimensional volume (Fig. 1). These three-dimensional data sets can be used for both quantitative descriptions of the impression replica as well as for visualization by specifying thickness as a function of colour (Fig. 2). Linear regression of the known calibration thicknesses versus micro-CT output appears in Fig. 3 (TableCurve 2D, v. 5.01, Systat Software, Inc., Richmond, CA, USA). Confidence intervals (95%) are very tight and the linear correlation is strong with a slope of 0.967 (r2 = 0.99). The distribution of thicknesses in the pre-cementation space replica is represented in Fig. 4 as volume percentages in each of the 12 thickness groups.
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Fig. 1 – 2D slice of pre-cementation space impression material (magnification bar = 1 mm).
3.2.
Quantitative optical analysis
Optical density data (as pixel values) versus calibration set thicknesses appear in Fig. 5 along with the calibration curve-fit by the MetaMorph1 software. As expected, this relationship is a natural logarithmic function of thickness (r2 = 0.95). This data was curve-fit (to the same equation used by MetaMorph1) to generate the 95% confidence intervals in Fig. 5 (TableCurve 2D). Fig. 6 shows the ‘‘raw’’ optical image of the cementation space ‘‘fillet’’ for orientation and the corresponding MetaMorph1 colour thickness colour map. Four different regions of clinical interest were identified (see Fig. 6 caption) that also
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Fig. 2 – 3D representation of pre-cementation space impression material.
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Fig. 3 – Linear regression analysis of micro-CT thickness measurements versus known thicknesses from micrometre.
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Fig. 4 – Pre-cementation space impression divided into 12 thickness groups, as measured by micro-CT, by the volume percentage for each group.
appeared to have different thicknesses. These four areas were used for statistical and comparative analysis. Fig. 7 (comparable to Fig. 4 for micro-CT) shows the overall distribution of thickness in six groups. The optical analysis with MetaMorph1 easily allowed regions to be defined which provided interesting additional information as examined in Section 4. Cumulative percent area by thickness for these four regions is graphed in Fig. 8.
4.
Discussion
The results of this investigation indicate that both techniques can be used to quantitatively measure impression replicas for pre-cementation space, each having limitations and advantages. An overview comparing the characteristics and capabilities of both techniques is presented in Table 1. In general,
Fig. 5 – Calibration curve of known thickness (mm) versus transmitted light (pixels values). This logarithmic function based on this calibration curve was applied to pixel values measured from the pre-cementation space ‘‘fillet’’ to generate optical thickness data (MetaMorphW).
micro-CT measurements were characterized by much tighter 95% confidence intervals than found for the optical imaging method (Fig. 3 versus Fig. 5) and was capable of collecting data over a larger thickness range. The different results (ranges) in Fig. 4 versus Fig. 7 likely resulted from true thickness differences in the two replicas rather than from technique variations, since both techniques were independently validated with micrometre-fabricated calibration sets. Whilst the optical technique is probably superior below 15 mm, its upper limit was not much greater than 100 mm and the ability to distinguish amongst specimens decreased towards this upper limit (e.g., plateau in Fig. 5 and increasing width of 95% confidence boundaries). Pixel values used over thicknesses between 10 mm and 100+ mm ranged only between 110 and 125 (of the 0–255 full scale). The range of values available for distinguishing thickness may have been somewhat limited since the camera used 8-bit image coding and all pixel data were collapsed into greyscale data. Increasing the range of values coding for thickness might tighten confidence intervals, allow for higher discrimination, and broaden the measurable thickness range. This might be possible using a 16-bit B&W camera instead of the 8-bit colour camera utilized in this work. MetaMorph1 Imaging System software can be used more easily to identify specific areas for thickness, such as those labelled in Fig. 6b. In addition, all regions are automatically orthogonal (i.e. 908 to the slice plane). The colour-coded image from MetaMorph1 provided a valuable qualitative visual sense of thickness variations by area and these areas can be separately used for calculating mean statistics. The technique of ‘‘filleting’’ the specimen so that it was held completely flat between microscope slides for measurement may become problematic when preparing bicuspid and molar specimens. In addition, the calibration set must be imaged alongside for this technique.
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Fig. 6 – (a) Anatomy of a cementation space ‘‘fillet’’ for orientation (cut made on insical edge extended down mesial and distal interproximal surfaces). (b) Regions of interest as defined for measurement: 1 = cervical third of crown; 2 = lingual slope between cervical third and lingual fossa; 3 = incisal 2/3 of labial wall; 4 = lingual fossa.
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Fig. 7 – Pre-cementation space impression divided into six thickness groups, as measured by MetaMorphW, by the surface area percentage for each group.
The micro-CT technique provides large volumetric data sets which can be used for research and teaching purposes. Thickness measurements and mean statistics typically are automatically available in accompanying analysis software.
Fig. 8 – Cumulative area by thickness for each of the four pre-cementation space areas defined above in Fig. 6b.
The result is highly precise over a wide thickness range. One thickness validation set per impression material is all that should be required to establish segmentation parameters, and is capable of resolving thickness to approximately 10 mm.
Table 1 – Summary of characteristics and capabilities of the two techniques. Micro-CT Digital 3D data set Only one calibration set needed for validation of thickness data per impression material Mean statistics are automatically available Specimen preparation is similar for any tooth Can measure precisely over wider thickness range (10 mm to >1 mm) Requires instrument availability
Quantitative optical analysis All regions are automatically quantified (due to physical ‘‘filleting’’ of specimens) Calibration set required in field-of-view for each specimen Mean statistics need to be calculated separately from pixels-per-thickness data for each region of interest Specimen preparation is more extensive for posterior teeth Can measure thicknesses less than 10 mm but upper limit of approximately 100 mm (depending on optical density of impression material, intensity of light source and digital camera characteristics) Requires imaging software availability
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Micro-CT data provides opportunities for dramatic 3D visualization. Micro-CT can be expensive and time-consuming regarding both scanning time for high spatial resolution needed as well as for volumetric analysis. Both techniques require either technician assistance or a learning curve for optimum software utilization.
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5.
Conclusions
Both micro-CT and an optical imaging method can be used to quantify the thickness of pre-cementation space impressions. Each has advantages and limitations but either technique has the potential for use as part of clinical studies or research into CAD/CAM protocol optimization.
Acknowledgement Authors gratefully acknowledge financial and materials support from 3M ESPE (Seefeld, Germany) that allowed this comparative analysis project to be undertaken.
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